This invention relates to a three-phase transformer which is used for a cycloconverter having a conversion frequency lower than a source frequency. More particularly, it relates to a three-phase transformer for a cycloconverter which does not undergo D.C. excitation even when an output frequency is set at 1/2 of an input frequency.
Heretofore, cycloconverters which can convert a source frequency of 50 Hz-60 Hz into an output frequency of several to 20 Hz have been employed for speed control on electric railways, in induction motors and the like.
FIG. 3 is a connection diagram showing a prior-art three-phase transformer system for a cycloconverter illustrated in, for example, "Denki-Kogaku Handbook (Handbook Of Electrical Engineering)," (issued by Denki Gakkai in 1971), page 714, Section 6. 4. 3, FIG. 168.
Referring to FIG. 3, each of the three three identical-phase transformers 1-3 includes a primary winding 4, and secondary windings 5 and 6 magnetically coupled therewith. Although depicted as a single winding in FIG. 3, each of the primary winding 4 and the secondary windings 5 and 6 is composed of three windings corresponding to respective phases U, V and W as will be stated later.
Three cycloconverter circuits of identical arrangement 7-9 are individually connected to the respective three-phase transformers 1-3. Each of the cycloconverter circuits 7-9 includes a positive group converter 10 and a negative group converter 11 which are respectively formed of thyristor circuits, and a circulating current reactor 12 which is connected in series with both the converters 10 and 11. In addition, three-phase outputs Iu-Iw from the respective cycloconverter circuits 7-9 are supplied to a three-phase induction motor (not shown).
FIG. 4 is a side sectional view showing the winding structure of one of the three-phase transformers in FIG. 3.
Referring to FIG. 4, the iron core 13 of a three-phase three-leg structure has three main legs 13U-13W which correspond to the U-phase, V-phase and W-phase, respectively. The primary windings 4U-4W of the respective phases individually wound around the main legs 13U-13W constitute the primary winding 4. The secondary windings 5U-5W and 6U-6W of the respective phases wound around the corresponding main legs 13U-13W constitute the secondary windings 5 and 6.
Each of the primary windings 4U-4W of the respective phases is divided into two sets. These sets are excited in parallel, and one of them is magnetically coupled to the secondary windings 5U-5W, while the other to the secondary windings 6U-6W.
The respective phases of each of the secondary windings 5 and 6 are delta-connected or star-connected to construct corresponding terminals which deliver three-phase signals U1-W1 and U2-W2. By way of example, the secondary winding 5U of the U-phase, the secondary winding 5V of the V-phase and the secondary winding 5W of the W-phase are delta-connected so as to deliver the three-phase outputs U1-W1 from the respective nodes of the delta connection.
Now, there will be described the operation of the prior-art three-phase transformer for the cycloconverter shown in FIGS. 3 and 4.
First, when three-phase currents IU-IW at a source frequency of 60 Hz are supplied to the primary winding 4 of the three-phase transformer 1, the primary windings 4U-4W of the respective phases are excited, and the three-phase signals U1-W1 and U2-W2 are respectively delivered from the corresponding secondary windings 5U-5W and 6U-6W. The three-phase signals U1-W1 derived from one secondary winding 5 are supplied to the positive group converter 10, while the three-phase signals U2-W2 derived from the other secondary winding 5 are supplied to the negative group converter 11.
The respective three-phase signals U1-W1 and U2-W2 are subjected to rectification and duty factor controls by thyristors included in the corresponding converters 10 and 11. Further, the resulting signals are converted into a desired frequency by the circulating current reactor 12 so as to produce the signal-phase output Iu of the U-phase.
Likewise, three-phase signals U3-W3 and U4-W4 from the three-phase transformer 2 are converted into the signal-phase output Iv of the V-phase by the cycloconverter circuit 8, and three-phase signals U5-W5 and U6-W6 from the three-phase transformer 3 are converted into the signal-phase output Iw of the W-phase by the cycloconverter circuit 9. The single-phase output Iu-Iw form three-phase outputs having phase differences of 120.degree. from one another, and are used from the speed control of the induction motor.
FIG. 5 is a waveform diagram for explaining the process in which one of the single-phase outputs is obtained. When an A.C. input voltage as shown in FIG. 5 is supplied as a three-phase current IU-IW, signals depicted as output voltages (hatched parts) are provided from the respective converters 10 and 11. These output voltages are accordingly derived through the circulating current reactor 12, whereby a single-phase output as indicated by an output fundamental wave voltage is obtained. In this case, the output frequency fo of each of the single-phase outputs Iu-Iw becomes 1/3 of the input frequency fi of the three-phase currents IU-IW. Therefore, assuming by way of example that the input frequency fi is 60 Hz, the output frequency fo is 20 Hz.
With the operation control system employing such three-phase outputs Iu-Iw, however, when the output frequency fo is about 1/2 of the input frequency fi, D.C. components are respectively generated in the three-phase signals U1-W1, . . . and U6-W6, These D.C. components also add to the three-phase currents IU-IW to be supplied to the primary windings 4U-4W of each of the three-phase transformers 1-3. Accordingly, each of the three-phase transformers 1-3 undergoes D.C. excitation, The main legs 13U-13W of the iron core 13 become saturated, with the result that the primary windings 4U-4W and the secondary windings 5U-5W and 6U-6W wound around these main legs 13U-13W are adversely affected electromagnetically and mechanically by overheating of the iron core 13, rush currents, etc.
In order to prevent this drawback, the system has heretofore been operated so that the output frequency fo for the speed control may have the following relation to the input frequency fi: EQU f&lt;fi/2
fo may be 0-25 Hz or so for a 60 Hz fi. In a case where the speed control range of a load such as an induction motor is to be expanded, speed changes have been performed using gears.
As thus far described, prior-art three-phase transformers for cycloconverters have produced the single-phase ouputs Iu-Iw of the respective phases by the use of the three three-phase transformers 1-3. Accordingly, there has been the problem that, when it is intended to obtain the three-phase outputs Iu-Iw whose output frequency fo is higher than 25 Hz, the D.C. excitation causes magnetic saturation in the iron cores 13 of the respective three-phase transformers 1-3 and renders the operation difficult. Another problemhas been that, when the speed range of the controlled system is to be broadened, gears must be used incurring increases in the cost of the overall apparatus and increases in labor for the maintenance of the moving parts.